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FIGURE 2.7 Voltage swell due to step load rejection. The nominal 480-V generator bus experienced a rise to 541 V that lasted for approximately 18 cycles. where Vmax and Vmin represent the change in voltage over the nominal voltage Vnom. For example, if the voltage in a circuit rated at 120 V nominal changed from 122 to 115 V, the flicker is given by: fv = 100 × (122 – 115)/120 = 5.83% In the early stages of development of AC power, light flicker was a serious problem. Power generation and distribution systems were not stiff enough to absorb large fluctuating currents. Manufacturing facilities used a large number of pumps and compressors of reciprocating design. Due to their pulsating power requirements, light flicker was a frequent problem. The use of centrifugal- or impeller-type pumps and compressors reduced the flicker problem considerably. The flicker problems were not, for the most part, eliminated until large generating stations came into service. Light flicker due to arc furnaces requires extra mention. Arc furnaces, commonly found in many industrial towns, typically use scrap metal as the starting point. An arc is struck in the metal by applying voltage to the batch from a specially constructed furnace transformer. The heat due to the arc melts the scrap metal, which is drawn out from the furnace to produce raw material for a variety of industrial facilities. Arc furnaces impose large electrical power requirements on the electrical system. © 2002 by CRC Press LLC 109-2nd st. S (Unit 429) Phase A RMS Voltage. Feb 07 2000 14:34:12 121V 120V 119V 118V 117V 14:32 Feb 07, 2000 17 seconds/div. 14:34 Feb 07, 2000 FIGURE 2.8 Voltage changes during elevator operation in a residential multiunit complex. The rate of voltage change causes perceptible light flicker. The current drawn from the source tends to be highly cyclic as arcs are repeatedly struck and stabilized in different parts of the batch. The voltage at the supply lines to an arc furnace might appear as shown in Figure 2.9. The envelope of the change in voltage represents the flicker content of the voltage. The rate at which the voltage changes is the flicker frequency: ∆V = Vmax – Vmin Vnom = average voltage = (Vmax + Vmin)/2 f = 2 × (Vmax – Vmin) × 100/(Vmax + Vmin) Normally, we would use root mean square (RMS) values for the calculations, but, assuming that the voltages are sinusoidal, we could use the maximum values and still derive the same results. It has been found that a flicker frequency of 8 to 10 Hz with a voltage variation of 0.3 to 0.4% is usually the threshold of perception that leads to annoyance. Arc furnaces are normally operated with capacitor banks or capacitor bank/filter circuits, which can amplify some of the characteristic frequency harmonic currents generated by the furnace, leading to severe light flicker. For arc furnace © 2002 by CRC Press LLC VOLTAGE ENVELOPE V(MIN) V(MAX) V FIGURE 2.9 Typical arc furnace supply voltage indicating voltage fluctuation at the flicker frequency. applications, careful planning is essential in the configuration and placement of the furnace and the filters to minimize flicker. Very often, arc furnaces are supplied by dedicated utility power lines that are not shared by other users. This follows from the principle that as the voltage source becomes larger (lower source impedance), the tendency to produce voltage flicker due to the operation of arc furnaces is lessened. Low-frequency noise superimposed on the fundamental power frequency is a power quality concern. Discussion of this phenomenon is included in this chapter mainly because these are slower events that do not readily fit into any other category. Low-frequency noise is a signal with a frequency that is a multiple of the fundamental power frequency. Figure 2.10 illustrates a voltage waveform found in an aluminum smelting plant. In this plant, when the aluminum pot lines are operating, power factor improvement capacitors are also brought online to improve the power factor. When the capacitor banks are online, no significant noise is noticed in the power lines. When the capacitor banks are turned off, noise can be found on the voltage waveform (as shown) because the capacitor banks absorb the higher order harmonic frequency currents produced by the rectifiers feeding the pot lines. In this facility, the rest of the power system is not affected by the noise because of the low magnitudes. It is conceivable that at higher levels the noise could couple to nearby signal or communication circuits and cause problems. Adjustable speed drives (ASDs) produce noise signals that are very often troublesome. The noise frequency generated by the ASDs is typically higher than the harmonic frequencies of the fundamental voltage. Because of this, the noise could find its way into sensitive data and signal circuits unless such circuits are sufficiently isolated from the ASD power lines. © 2002 by CRC Press LLC FIGURE 2.10 Low-frequency noise superimposed on the 480-V bus after switching off the capacitor bank. 2.3 CURES FOR LOW-FREQUENCY DISTURBANCES Power-frequency or low-frequency disturbances are slow phenomena caused by switching events related to the power frequency. Such disturbances are dispersed with time once the incident causing the disturbance is removed. This allows the power system to return to normal operation. Low-frequency disturbances also reveal themselves more readily. For example, dimming of lights accompanies voltage sag on the system; when the voltage rises, lights shine brighter. While low-frequency disturbances are easily detected or measured, they are not easily corrected. Transients, on the other hand, are not easily detected or measured but are cured with much more ease than a low-frequency event. Measures available to deal with lowfrequency disturbances are discussed in this section. 2.3.1 ISOLATION TRANSFORMERS Isolation transformers, as their name indicates, have primary and secondary windings, which are separated by an insulating or isolating medium. Isolation transformers do not help in curing voltage sags or swells; they merely transform the voltage from a primary level to a secondary level to enable power transfer from one winding to the other. However, if the problem is due to common mode noise, isolation transformers help to minimize noise coupling, and shielded isolation transformers © 2002 by CRC Press LLC V 1 V 2 SHIELD PS C C SS SECONDARY PRIMARY C CAPACITANCE BETWEEN THE PRIMARY AND THE SHIELD AND THE SECONDARY AND SHIELD FORM A POTENTIAL DIVIDER REDUCING V2 TO A LOW LEVEL C PG V 2 = V XC 1 C PS + C PS SS SG G FIGURE 2.11 Common mode noise attenuation by shielded isolation transformer. can help to a greater degree. Common mode noise is equally present in the line and the neutral circuits with respect to ground. Common mode noise may be converted to transverse mode noise (noise between the line and the neutral) in electrical circuits, which is troublesome for sensitive data and signal circuits. Shielded isolation transformers can limit the amount of common mode noise converted to transverse mode noise. The effectiveness with which a transformer limits common mode noise is called attenuation (A) and is expressed in decibels (dB): A = 20 log (V1/V2) where V1 is the common mode noise voltage at the transformer primary and V2 is the differential mode noise at the transformer secondary. Figure 2.11 shows how common mode noise attenuation is obtained by the use of a shielded isolation transformer. The presence of a shield between the primary and secondary windings reduces the interwinding capacitance and thereby reduces noise coupling between the two windings. Example: Find the attenuation of a transformer that can limit 1 V common mode noise to 10 mV of transverse mode noise at the secondary: A = 20 log (1/0.01) = 40 dB Isolation transformers using a single shield provide attenuation in a range of 40 to 60 dB. Higher attenuation may be obtained by specially designed isolation © 2002 by CRC Press LLC transformers using multiple shields configured to form a continuous enclosure around the secondary winding. Attenuation of the order of 100 dB may be realized with such techniques. 2.3.2 VOLTAGE REGULATORS Voltage regulators are devices that can maintain a constant voltage (within tolerance) for voltage changes of predetermined limits above and below the nominal value. A switching voltage regulator maintains constant output voltage by switching the taps of an autotransformer in response to changes in the system voltage, as shown in Figure 2.12. The electronic switch responds to a signal from the voltage-sensing circuitry and switches to the tap connection necessary to maintain the output voltage constant. The switching is typically accomplished within half of a cycle, which is within the ride-through capability of most sensitive devices. Ferro-resonant voltage regulators are static devices that have no moving components. They operate on the principle that, in a transformer, when the secondary magnetic circuit is operating in the saturation region the secondary winding is decoupled from the primary and therefore is not sensitive to voltage changes in the primary. The secondary winding has a capacitor connected across its terminals that V in V out VOLTAGE SENSOR FIGURE 2.12 Tap-changer voltage regulator. © 2002 by CRC Press LLC forms a parallel resonant circuit with the inductance of the secondary winding. Large magnetic fields are created in the magnetic core surrounding the secondary windings, thereby decoupling the secondary winding from the primary. Typically ferro-resonant transformer regulators can maintain secondary voltage to within ±0.5% for changes in the primary voltages of ±20%. Figure 2.13 contains the schematic of a ferroresonance transformer type of voltage regulator. Ferro-resonance transformers are sensitive to loads above their rated current. In extreme cases of overload, secondary windings can become detuned, at which point the output of the transformer becomes very low. Voltage sags far below the rated level can also have a detuning effect on the transformer. Within the rated voltage and load limits, however, the ferro-resonance transformer regulators are very effective in maintaining fairly constant voltage levels. 2.3.3 STATIC UNINTERRUPTIBLE POWER SOURCE SYSTEMS Static uninterruptible power sources (UPSs) have no rotating parts, such as motors or generators. These are devices that maintain power to the loads during loss of PRIMARY WINDING COMPENSATING WINDING V in CORE SHUNT C V out CONSTANT VOLTAGE RESONANT WINDING (SECONDARY) FIGURE 2.13 Ferro-resonant transformer. © 2002 by CRC Press LLC normal power for a duration that is a function of the individual UPS system. All UPS units have an input rectifier to convert the AC voltage into DC voltage, a battery system to provide power to loads during loss of normal power, and an inverter which converts the DC voltage of the battery to an AC voltage suitable for the load being supplied. Depending on the UPS unit, these three main components are configured differently. Static UPS systems may be broadly classified into offline and online units. In the offline units, the loads are normally supplied from the primary electrical source directly. The primary electrical source may be utility power or an in-house generator. If the primary power source fails or falls outside preset parameters, the power to the loads is switched to the batteries and the inverter. The switching is accomplished within half of a cycle in most UPS units, thereby allowing critical loads to continue to receive power. During power transfer from the normal power to the batteries, the loads might be subjected to transients. Once the loads are transferred to the batteries, the length of time for which the loads would continue to receive power depends on the capacity of the batteries and the amount of load. UPS units usually can supply power for 15 to 30 min, at which time the batteries become depleted to a level insufficient to supply the loads, and the UPS unit shuts down. Some offline UPS system manufacturers provide optional battery packs to enhance the time of operation of the units after loss of normal power. In online UPS units, normal power is rectified into DC power and in turn inverted to AC power to supply the loads. The loads are continuously supplied from the DC bus even during times when the normal power is available. A battery system is also connected to the DC bus of the UPS unit and kept charged from the normal source. When normal power fails, the DC bus is supplied from the battery system. No actual power transfer occurs during this time, as the batteries are already connected to the DC bus. Online units can be equipped with options such as manual and static bypass switches to circumvent the UPS and supply power to the loads directly from the normal source or an alternate source such as a standby generator. An offline unit is shown in Figure 2.14, and an online unit in Figure 2.15. Two important advantages of online UPS units are because: (1) power is normally supplied from the DC bus, the UPS unit in effect isolates the loads from the source which keeps power system disturbances and transients from interacting with the loads, and (2) since power to the loads is not switched during loss of normal power, no switching transients are produced. As might be expected, online UPS systems cost considerably more than offline units. The output voltage of static UPS units tends to contain waveform distortions higher than those for normal power derived from the utility or a generator. This is due to the presence of the inverter in the output section of the UPS system. For some lower priced UPS units, the distortion can be substantial, with the waveform resembling a square wave. Figure 2.16 shows the output waveform of a UPS unit commonly used in offices to supply computer workstations. More expensive units use higher order inverter sections to improve the waveform of the output voltage, as shown in Figure 2.17. It is important to take into consideration the level of susceptibility of the loads to waveform distortion. Problems attributed to excessive voltage distortion have been noticed in some applications involving medical electronics and voice communication. © 2002 by CRC Press LLC SWITCH #1 NORMAL AC POWER CIRCUIT BREAKER AC OUPUT RECTIFIER SWITCH #2 INVERTER BATTERY VOLTAGE AND CURRENT SENSOR FIGURE 2.14 Offline uninterruptible power source (UPS) system. ALTERNATE SOURCE PREFERRED SOURCE STATIC BYPASS SWITCH MANUAL BYPASS CHARGER INVERTER DC BUS BATTERY BANK VOLTAGE AND CURRENT SENSOR FIGURE 2.15 Online uninterruptible power source (UPS) system. 2.3.4 ROTARY UNINTERRUPTIBLE POWER SOURCE UNITS Rotary UPS (RUPS) units utilize rotating members to provide uninterrupted power to loads, as shown in Figure 2.18. In this configuration, an AC induction motor drives an AC generator, which supplies power to critical loads. The motor operates from normal utility power. A diesel engine or other type of prime mover is coupled to the same shaft as the motor and the generator. During normal operation, the diesel engine is decoupled from the common shaft by an electric clutch. If the utility power © 2002 by CRC Press LLC FIGURE 2.16 Output voltage waveform from an offline uninterruptible power source (UPS) system. The ringing during switching is evident during the first cycle. fails, the prime mover shaft is coupled to the generator shaft and the generator gets its mechanical power from the prime mover. The motor shaft is attached to a flywheel, and the total inertia of the system is sufficient to maintain power to the loads until the prime mover comes up to full speed. Once the normal power returns, the induction motor becomes the primary source of mechanical power and the prime mover is decoupled from the shaft. In a different type of RUPS system, during loss of normal power the AC motor is supplied from a battery bank by means of an inverter (Figure 2.19). The batteries are kept charged by the normal power source. The motor is powered from the batteries until the batteries become depleted. In some applications, standby generators are used to supply the battery bank in case of loss of normal power. Other combinations are used to provide uninterrupted power to critical loads, but we will not attempt to review all the available technologies. It is sufficient to point out that low-frequency disturbances are effectively mitigated using one of the means mentioned in this section. 2.4 VOLTAGE TOLERANCE CRITERIA Manufacturers of computers and data-processing equipment do not generally publish data informing the user of the voltage tolerance limits for their equipment. An agency © 2002 by CRC Press LLC